This invention relates to apparatus for blocking a d.c. component of a signal. The invention relates, particularly, though not exclusively, to apparatus associated with an a.c. signal feed into a hazardous environment, for preventing a spark or arc.
The provision of signals to and from hazardous environments in an intrinsically safe manner is desirable in a number of industries including, for example, the oil and gas industries, the chemicals industry, and environments such as flour mills where dust explosions are possible. In general, equipment for use in such environments must undergo rigorous testing and pass strict criteria to be certified for this type of use.
If a high frequency a.c. signal needs to be fed into such an environment, it is important that d.c. potentials are not transmitted at the same time. This is because such potentials might cause an arc or spark discharge if they are brought close to an electrically conductive object at earth potential. This could result in an explosion in a hazardous flammable environment.
To overcome this problem a number of solutions have been devised. For example, the signal may be fed into the hazardous region as a modulated light beam by way of a fibre-optic light guide. As an alternative, the signal may be radiated into the hazardous environment as an electromagnetic wave from a first (transmitting) antenna to a second (receiving) antenna through a dielectric window.
Such solutions can be expensive to implement and have other associated problems such as, for example, the requirement to convert the required signal to and from excitations in different media resulting in lower efficiency, poorer reliability, and increased power consumption.
It is an object of the present invention to provide an improved apparatus for use in the above mentioned applications.
According to a first aspect of the invention there is provided an apparatus for blocking a d.c. component of a signal, comprising an electrically conductive signal path having a gap in it preventing direct current flow across the gap, and an electrically conductive element spaced from the path by a body of dielectric material, the element being located and dimensioned such that in use an a.c. signal is coupled from the signal path into the element at one side of the gap, and from the element into the signal path at the other side of the gap, characterised in that the apparatus further comprises an electrically conductive earthing path, one end of which is connected to earth, the other end of which is connected to the signal path, the earthing path having a length being an odd multiple of a quarter wavelength of a signal of a given frequency, the earthing path providing a short circuit to earth for signal components having frequencies different from the given frequency or harmonics of the given frequency.